The invention is specifically directed to a method for cleaning perforated, slotted and wire-wrapped well liners which become plugged with foreign material by means of devices using high velocity liquid jets. However, it will be understood that in certain instances the inventive method can be applied to cleaning pipes in general and as used herein the term "pipe" shall include well liners.
In the well producing art, it is customeray to complete wells, such as water, oil, gas, injection, geothermal, source, and the like, by inserting a metallic well liner adjacent a fluid-producing formation. Openings in the well liner provide passage-ways for flow of fluids, such as oil or water and other formation fluids and material from the formation into the well for removal to the surface. However, the openings, which, for example, may be slots preformed on the surface or perforations opened in the well, will often become plugged with foreign material, such as products of corrosion, sediment deposits and other inorganic or hydrocarbon complexes. The amount of energy which is needed to remove the different types of foreign matter varies depending upon the material. This energy can be predetermined for each and every case encountered in the field.
Since removal and replacement of the liner is costly, various methods have been developed to clean plugged openings including the use of jetted streams of liquid. The use of jets was first introduced in 1938 to directionally deliver acid to dissolve carbonate deposits. Relatively low velocities were used to deliver the fluid. However, this delivery method did improve the results of acidizing. In about 1958 the development of tungsten carbide jets permitted including abrasive material in a liquid which improved the ability of a fluid jet to do useful work. The major use of abrasive jetting has been to cut notches in formations and to cut and perforate casing to assist in the initiation of hydraulically fracturing a formation. The abrasive jetting method requires a large diameter jet orifice. This large opening required an unreasonably large hydraulic power source in order to do effective work. The use of abrasives in the jet stream permitted effective work to be done with available hydraulic pumping equipment normally used for cementing oil wells. However, the inclusion of abrasive material in a jet stream was found to be an ineffective perforation cleaning method in that it enlarged the perforation which destroyed the perforation's sand screening capability.
More recently, Chevron Research Company disclosed a method and apparatus for directionally applying high pressure jets of fluid to well liners in a number of U.S. patents. These patents were U.S. Pat. Nos. 3,720,264, 3,811,499, 3,829,134, 3,850,241 and 4,088,191, which are herein incorporated by reference.
The assignee of the subject application is a licensee of the Chevron system and developed a cleaning operation and device pursuant to the Chevron disclosures. This system employed a jet carrier of about 6 feet in length having 8 jet nozzles widely spaced along its length. The nozzles were threadably mounted on extensions which were in turn welded to the jet carrier. A fixed tri-blade pilot bit was affixed to the lower end of the jet carrier. The jet carrier was attached to a tubing string that could be reciprocated and rotated within the well bore. As the carrier was moved and rotated adjacent the liner, the nozzles directed jet streams which contacted and cleaned the liner.
This design, although an improvement over prior designs, developed a number of problems. No relationship between the vertical and rotational speeds was known which would ensure efficient and complete liner coverage by the fluid streams. Thus, if the rotational speed was held constant and the vertical speed decreased, the streams would cover the liner a multiplicity of times. If vertical speed were increased the streams would miss areas of the target. Conversely, if vertical speed were held constant and rotational speed increased, complete coverage was achieved but with insufficient energy to remove the material. If rotational speed was decreased, gaps would occur in the liner area covered by the streams.
In an attempt to solve these problems, Applicant developed its own jet carrier assembly fully described in co-pending application Ser. No. 195,303 filed Oct. 7, 1980, now U.S. Pat. No. 4,349,073 which is herein incorporated by reference.
This assembly has between about 8 and 16 nozzles spaced along its length. An equation is used to determine the jet stream track pattern against the liner for a jet tool having a given nozzle number and spacing and which is rotated and moved vertically at selected speeds. The spacing between the tracks is then calculated from this track pattern. Comparing this spacing with the known width of the jet streams determines the amount of coverage the streams provide on the liner. Using this equation, a set of rotational and vertical speeds of a constant ratio were determined which would provide jet streams having theoretical double coverage over all points on the liner when using 16 nozzles.
This design and method allows the use of greater vertical and slower rotational speeds without producing gaps in the cleaning coverage. Moreover, the decreased time to cover a given interval vertically by the virtue of increasing the vertical speed, reduces the amount of overall time necessary to do a given job, while at the same time covering all points on the liner with jet streams at least once. The new design which offered 13 different standard tool body sizes kept the nozzle within a more effective range of the target, permitting delivery of the fluid uniformly against the liner slots and perforations with an average of two to five times the energy of the Chevron system.
Although this design was a major advance in the art, it did not take into account a number of field factors. First, the design did not attempt to relate the rotational and vertical speeds to the diameter of the liner. This is important because for given values of rotational and vertical speeds, the tangential velocity of the fluid streams increases with increasing liner diameter. As the tangential velocity increases, the cleaning energy of the fluid streams decreases. With large liners, the cleaning energy can become insufficient to remove foreign matter, if corrective steps are not taken, even though the streams are striking each point on the liner twice. Thus, the prior systems did not relate the energy needed to clean the liner to the total energy actually being produced by the fluid streams. This total energy is dependent upon, not only the particular values of rotational and vertical speeds selected, but also the decrease in power of the streams as they travel between the nozzle and the liner. This power drop is in turn dependent upon the distance between the nozzle and the liner, i.e., the stand-off distance.
Thus, although the prior system insured theoretical complete coverage of the liner it did not insure that the particular rotational and vertical speeds would produce the required energy to clean foreign matter from a liner of a given size. Nor did the design take into account the energy lost by the streams between the nozzles and the liner.
As a result, a strong need continues to exist for a method of cleaning well liners which can consistently and accurately produce a given energy at the liner to clean the particular foreign material present in a controllable, economical field operation.